Production of impregnated fiber mats



United States Patent-C 2,813,806 PRODUCTION OF IMPREGNATED FIBER MATS Otis R. Videen, Cloquet, Minn., assignor to Wood Conversion Company, St. Paul, Minn., a corporation of Delaware No Drawing. Application April 5, 1955, Serial No. 499,505

4 Claims. (Cl. 117-140) A valuable characteristic is that in drying the mat impregnated with them, such starch does not migrate from the interior to the surface exposed for drying. However, in continuous commercial operations using such nonmigrating starches in an impregnating bath for the mat, the starch alters its viscosity -characteristics as the bath ages from initial formation. As a result, the continuous process does not operate on a standardized basis and the product varies with the aging of the starch. Therefore, a search was instituted to find similar low-cost binders which form stable dispersions not subject to change in aging and in processing as used in a commercial continuous operation. Numerous such binders have been found. They include certain native starches, and modified starches and amylaceous derivatives thereof. However, in using such stable materials in the same i manner as using the non-migrating pearl starch, it was thereof is suitable for impregnation. A plurality of superimposed impregnated thin layers may be dried, and as a result of the process the several layers become integrated. Accordingly, the term fiber mat is herein employed to designate a web of material subjected to impregnation by the present invention, whether it is one or more layers.

The substance employed for impregnating the fiber mat is one which acts as binder and size for the fibers and which imparts strength and stiffness in desired degrees.

Furthermore, it is substance which is water-dispersible,

being a starch or amylaceous products deriving from starch.

The impregnation of fiber mats with such aqueous dispersions is well known, and is commonly elfected by a suitable operation to load the mat to a desired content of water and dispersion solids for subsequent drying. For economy of commercial practice, the dispersion as applied to the mat must have stability so that it will not change while held in the bath, and it must be non-substantive to the fiber so that it will not deposit the dispersed binder merely by contact upon impregnation. In consequence, it maintains its stability and its mobility in the impregnating liquid after it is introduced into the fiber mat.

The present invention involves the property of a material to impart to an aqueous solution or dispersion of it, a measurable viscosity. The concentration of the dispersed material predetermines the viscosity, other conditions being the same, such as temperature. The viscosity of the dispersion may change with temperature. The viscosity change may be small or great relative to the change in temperature, and it may increase or decrease depending upon the material. Those properties of the material itself as a solid, which predetermine theviscosity of the aqueous solutions or dispersions and the change of I found that they migrate in drying the impregnated mat. Thus, the process operated by a standard procedure produced non-homogeneous and non-uniform products, which are relatively weaker and more flexible in the interior from loss of binder by migration, and relatively stronger and stiffer at the surface exposed to drying by reason of the migration of binder from the interior.

The present invention provides a remedy against such migration for minimizing or substantially preventing it, so that the nature of the products may be standardized, yet varied by control of the degree of migration.

It is the general object of the invention to produce improved fiber fabrics impregnated and bound with aqueous dispersions of certain starches and starch derivatives.

It is a particular object of the present invention to minimize the migration of dispersed amylaceous binder when drying fiber mats impregnated therewith.

It is a particular object of the invention to permit but to control migration of dispersed amylaceous binder solids in a manner to control certain characteristics of the product.

Various other and ancillary objects and advantages of the invention will become apparent from the following description and explanation of the invention.

The present process is herein described and explained with reference to the preferred practice with air-laid fiber felts, but it is to be understood that the invention is not thereby limited to such felts as the fiber mat to which the present invention relates.

By certain air-laying technique, a continuous stream of a suspension of fibers in air may be deposited on a moving foraminous member to form an unbonded felt of a fiber density in the range from 1 to 2 pounds per cu. ft. Such a web may be continuously removed and may require physical support because of its lack of strength in the unbonded condition. The web is preferably carried on a wire-mesh screen to provide such support, and to carry it into a liquid impregnating bath. The felt need not have a low density in the range from 1 to 2 pounds per cu. ft., and it may have a greater density and such that its felted condition holds the felt against breaking up in a liquid bath. However, where the possibility of disintegration exists, the felt is carried into and out of contact with impregnating liquid, for example, through a bath thereof, between two foraminous carriers, preferably wire-screen conveyers.

To insure uniform impregnation, an open mat is preferred, for example, a uniformly felted low-density web.

An open mat may be impregnated in a variety of ways. One method is to immerse it in an impregnating liquid bath under conditions to provide an expressible excess of the liquid in the mat, and then to express the excess. This may be done for any type of mat by housing it between two wire-mesh conveyers and passing the assembly into, through and out of an impregnating bath. The two conveyers preferably house the mat during the compression to express excess liquid, and thus they maintain the felted integrity of the mat, however loose it maybe. When expansion of the mat is desired on drying, the conveyers are separated from the mat, and it is dried without restraint.

Such an impregnated mat may be processed in a manner to meet specifications for a dry end-product, as to usage of binder, thickness and density, by exercising various controls. Since the process involves drying, water must be removed by evaporation. This may induce migration of the dispersed binder which has been introduced by the impregnation. By minimizing the extent of evaporation, migration may be minimized and controlled. According to the .present invention, the extent of evaporation is minimized by mechanically ex.- pressing water, with the additional advantage that costs are lessened in removing water. Accordingly, the wet product is reduced in water content by expressing excess impregnating liquid to the desired extent to leave a desired residual amount. In doing this, the wet mat itself is densified. On drying without restraint, the extent of migration is related to expansion of the mat on drying. The extent of wet compression in removing excess liquid may be used to control the amount of solids to be left in the'mat as ultimate binder therefor. Consequently, all these factors predetermine the concentration of dispersed solids to be provided in the original impregnating bath for a particular product. For ma.- terials of standardized quality, varying the concentration of dispersed solids in the impregnating liquid, and controlling the compression to remove liquid, permit predetermining the wet density of the mat and the residual content of impregnated solids, so that a product of predetermined binder or other treating content and of predetermined density mayibe formed on drying.

The wet compressed impregnated mat to be driedhas inherently a substantially uniform distribution of impregnating liquid. The drying may be effected by exposure of the wet mat at one or both faces to the atmosphere or other drying gaseous environment, at normal or elevated temperatures. The loss of water by vaporization at a face of the mat leads to a difiusion of liquid from inward regions of the mat toward such face. When the binder solids are such as will migrate with the liquid, there is a growing'content of dispersed or dissolved material at said face, as is well known.

Because it is desired to admit impregnating liquid into the mat and'then to express a fraction so admitted, a liquid is used which remains stable in the bath and especially in the mat, and one of which, the solids are mobile while in the liquid in the mat. Consequently, the residue of liquid in the mat after expressing excess is stable and must be non-substantive to thefiber so as not to deposit its solid content on the fibers. Hence, such solids are highly susceptible to migration as soon as evaporation begins.

Experience with many formulations of starchy binders in aqueous dispersions leads to the conclusion that during the evaporation each composition thickens to a critical concentration involving a viscosity factor, at which migration of dispersed solids is greatly reduced or ceases. That a viscosity factor is involvedis evident from the fact that when certain hydrophilic colloids which provide highly viscous sols are present, the .migration is minimized as the concentration of any onestlCh col, kid is increased, and also as the viscosity characteristics 4 of the a ous c l oi n re a a fixed conc ntr tion thereof.

The starchy binders, whether or not they migrate, are hydrophilic colloids, but their aqueous dispersions are characterized by decreasing viscosity as temperature increases. The increase of temperature from impregnating to drying has an adverse effect leading to migration by decrease of viscosity. But the loss of water by drying tends to increase the viscosity in compensation. Thus, concentration and temperature effects are involved, as well as the viscosity characteristics of a particular amylaceous binder.

Since there are practical limits of concentration for using each specific amylaceous binder in aqueous dispersion, the various ones may be relatively evaluated at some particular but arbitrary concentration suitable for all. For this purpose, various ones have been tested for viscosity at 5% concentration by weight in water dispersion and at F. By the Dudley viscosity, they have been classified as to their relative tendencies to migrate under the conditions hereinafter exemplified.

It has been found that where the measured viscosity is upwardly from-45 seconds, that migration is negligible. Pearl starch and someothers fall into this group. Those which have a viscosity of 40 seconds and below migrate disadvantageously. Those in the range from 40 to 45 seconds have inconsistent migration characteristics.

However, it is not intended to limit the amylaceous binders of the present invention by limiting viscosity characteristics, because migration is also predetermined by the concentration of it in the impregnating bath, and by the concentration of such binder solids in the wet mat to be dried.

Since the starchy binders which may migrate lead to lowered viscosity of the impregnating liquid as the temperature is increased for drying, the present invention contemplates the addition of small amounts of a hydrophilic colloid which exhibits a high viscosity at a very low, concentration.

A wide variety of hydrophilic agents which are colloidal-like or actual colloids and which form viscous aqueous sols at relatively low concentrations has been found useful to provide ingredients of an impregnating aqueous disperson of amylaceous binder, which agents are inert to the stability .of the liquid, which do not destroy the impregnating fluidity of the bath for the particular. mat, and which eventually minimize migration in evaporative drying. Among the materials which may be used are natural gums and synthetic materials. Examples of-natural gums are: guar, locust bean, agaragar, Irish moss, g m karaya, and tragacanth. Among the'synthetic materials are certain cellulose derivatives, especially methyl cellulose, and sodium carboxymethyl cellulose; hydrophilic organic polymers, such as polyacrylate salts, polyvinyl alcohol, copolymer of methyl vinyl ether and maleic anhydride, Carbopol 934 a trade-marked product of B. F. Goodrich Chemical Company, Cleveland, Ohio; and derivatives of natural material such asthe sodium and other water-soluble salts ofalginic acid.

In choosing such a hydrophilic agent for incorporation into an aqueous dispersion of amylaceous binder, it is one aim to minimizethe contentof the agent. Therefore, the ones which are preferred arerthose which function at concentrations not exceeding 1 part per parts by weight oftheimpregnating bath. The greater the viscosity characteristics of .;a particular agent, the less of it that is required. For example, the cellulose derivativesare commercially .avaiiablein various forms identified as having certain viscosity characteristics. One commercialsource of methyl cellulose determines its viscosity characteristics at 2% concentration in water at 20 C. For example, one; form X-showss4000 .cps. Another form Y shows 10 cps. Form X used in the present invention at 0.3% con- TABLE I Sodium salts of carboxymethyl cellulose CentipQises-at 25 C.

Identification CMC (Na) High v 1,-300-2, 20o CMC (Na) Med. v 300-600 ClVIC (Na) Low V The chemical compositions of the hydrophilic agents are not pertinent to the present invention, and the only criterion of their suitability is their viscosity characteristics. These vary so greatly that asuitable range of usage of one is an entirely unsuitable range for another. Any such agent may be used when some concentration of it in the range from 0.1% to 1% of the impregnatingbath is effective to maintain the impregnating fluidity of the bath for a particular mat, and also effective to minimize migration of amylaceous binder present with it in the bath. This yardstick will better be appreciated in discussion of actual comparisons.

Methyl cellulose has a special advantage over other hydrophilic agents in that the viscosity of its'solutions increases with increase in temperature. Advantage istherefore secured in heating the expressed mat to highertemperature for evaporation, in that viscosity increase to minimize migration may be achieved before any substantial .reduction in water content, and by virtue of increased temperature only. Hence, methyl cellulose is the more advantageous in use. However, it requires special manipulation to dissolve it to sol form, making other materials more desirable for practical purposes. The figures used herein following the listing Methocel for methyl cellulose in Table V represent the viscosity in centipoises at 2% concentration in water at 20 C.

To facilitate impregnation, wetting agents may be included in the bath. These may be cationic, nonionic or anionic. The cationic and anionic ones are chemically reactive and to some extent impose limitations on the presence of reactive materials either in the bath or in the mat. Because nonionic wetting agents are inert chemically, they are preferred for all kinds of fiber. From to 1% content of such is suflicient.

According to the present inventiongthere is presentin the impregnating liquid one or more ofsuch hydrophilic agents which are inert to the stability of the dispersion, and which are effecitve in drying to minimize migration of dispersed solids.

In drying a wet impregnated mat having dispersed amylaceous solids subject to undesired migration, the drying takes place in two major stages. In the'first stage, the liquid becomes more concentrated in dispersed solids, the more so near the drying face because of loss of water therefrom and because of migration of solids to it. Such tended in so impregnating and drying fiber mats. The

-critieal--.concentration is attained in the drying face before itis attained. inwardly therefrom..

The secondstage ofdnyinghinvolvesathe loss of water from the region of the fibers on-"Which lithe solids have 5 deposited. This causes diffusion of liquid to it from within, and where migration is not opposed, migratable dispersed particles move with the diffusing liquid toward the drying face, thus depleting the inner regions. By this mechanism, a Skll'kllke surface at one orrbothfaces' may be formed relative to the interior.

In carrying out the invention in comparative examples to evaluate the migration of starchy binder, four mats A, B, C and D Were used, which are :identified as follows:

TABLE II A. 85% cotton, 15% sulfite cellulose B. 25% cotton, 75% sulfite cellulose C. 70% cotton, 30% sulfite cellulose D. 50% cotton, 50% wool The following types of starchy binder were used:

TABLE III Dudley, Identification Seconds Viscosity S-l Oxidized Corn Starch (low) Oxidized Corn Starchtmedium). Oxidized Potato Starch. Dextrin Oxidized Oorn Starch oeccudabo P95 99? almanac Enzyme hydrolysates of corn starch have been used and these result in migration. However, owing to difliculties in controlling such hydrolyses, the results have not been reproducible and such hydrolysed starch may not be readily specified nor used for comparisons. However, their migration may be minimized by the use of the same hydrophilic agents as are used for the more standardized starches and their modified and amylaceous derivative forms.

Not only is the viscosity of the impregnating bath a factor, but also the mat. Its fiber content and its density are factors. Fibers vary in the wettability by water and by aqueous solutions, as well as in their holding capacity of such when wetted, and the history of the fibers, such as treatments in commercial uses, also vary these qualities. Therefore, mats varying in composition as to cotton, wool and wood cellulose, exhibit difierent degrees of migration for a given binder solution; This will be apparent in the data of Table IV.

The fiber mats were thoroughly impregnated with an expressible excess of liquid at room temperature andthe excess liquid was squeezed out by passage through rolls set so that the retained content of binder solids was 10 parts per 90 parts of original mat weight. Then the-mats were dried without restraint by exposure of both faces with forced air circulating at 250 to 300 F. g

In comparative tests for migration, wherein similar mats are wet-compressed to the same thickness with the same content of binder solids, it has been found that'the more the binder solids migrate, the thicker is the final mat on drying without restraint. The loss ofb'inder in the interior as a result of migration lets the fibers exert their natural resiliency, and the mat expands on drying. Therefore, the bonded strength of the mat at the center, and the final thickness, are indices of migration.

In Tables IV, V and VI there are given data indicating the extent of migration. Table IV indicates the binder, the mat, the hydrophilic agent and the data onmigration effects by inspection methods. Table V shows the data for making specimens of Examples 1 through 22. "In Table VI, the physical characteristics of those specimens .are given.

Free

inches Adhesion Stiffness,

Density,

TABLE VI Tensile Strength, 1bs./inch width Weight,

In Table VI, the value stretch is elongation by a standas percent increase from original length.

The value free stiffness in inches is a comparative value and is the length which may be horizontally projected under controlled conditions until a predetermined 30 flexure is evident.

inches Thickness,

Example .TABLBIV Percent Binder Bath Web AABBDDAAAAAAAAAAAOD No'nr 1.-Viscosity of impregnating bath in seconds at 80 F. Dudley pipette.

.Two series of impregnated mats were made, using the 25 ardized pulling of a sample at the break-point expressed mats C and D, each having the binder designated S-2 in amount of 7 parts per 93 parts of fiber, and a variety of hydrophilic thickeners for anti-migration efiect. Table V designates some processing factors, and Table VI the properties of the completed mats.

Viscosity (Note 1) g -y -f nwm m mmb ma .US.1L m .l n M om e mu em w uw wmm w mm CS C mnmm w 3m mmn am m NW-1H0 v h n a M e7f s ma m 3 6 Y6 P m m mw mmw m mo n me m wmwo m n mfl Sm h 1 SP n P a 0 m .lnou e o r e m m )w P ws m tyfli m e m h h O .0 B

mn mmemmm mw am mmsmnma n 8 w a mid a n o xn 0 m h o c m mz m 1 e m Sd a .mO h wcvta m c WH p v two r om e cSs s m h la fmn PW m P u M w w es dm Cm mtmnm m atm a .m m mhm mmc snm T 5.1 1 n Y8K 6 f mmme be 0 NW Pm 0 t 5 0 n0 6 7 054462019002004425 g 4 Percent in Solution Mat retaining 7 parts of binder S-Z per 93 parts of fiber Example Using impregnating bath containing 3% of binder 8-2 and t im re nati vb a 3 ontent 0 d NOTE 1.ln seconds at 80 F. Dudley pipette at 5% binder'concentra- 7 gz y p gg z c m Sec 8 S tion in impregnating bath.

TABLE VII Weight, Tensile Ad- Free Ex. (a) (b) Mat Thickness, lbs/M Strength, Stretch Density, hesio Stlfiness,

inches sq. ft. lbs/in. lbs/cu. it. grams inches width 36. 4 C 0. 068 29 1. 45 4. 6 5. 12 6. 1 0. 1 38. 8 O 0. 045 29 1. 65 3. 4 7. 73 25 9. 4 0.3 46 4 C 0. 042 30 3. 8 2.3 8. 56 65 11.0 0 36.4 D 0.155 79 2.9 2.0 6.12 8.5 9.6 0. 3 46. 4 D 0. 129 80 5. 2. 3 7. 43 12 11. 5

TABLE VIII Weight, Tensile Ad- Free Ex. (0) (6) Mat Thickness. lbs/M Strength, Stretch Density, hesion, Stiffness,

inches sq. it. lbs/1n. lbs/cu. ft. grams inches width Table VII shows the use of three impregnating baths in the same procedure to leave the same amount of binder solids, in mats C and D. With both mats, the migration was lessened by use of the CMC-HV as indicated by the increased adhesion, the decreased thickness and increased density.

The invention is not limited to the details given in the illustrative series above. For example, the impregnating liquid may contain amylaceous binder at concentrations greatly in excess of 5%, for example 10%. The porosity of the fiber mat, the particular binder used, the particular kind and amount of hydrophilic thickener, and the temperature of the impregnating liquid are among factors which predetermine the maximum concentration of binder for retaining an impregnating fluidity.

EXAMPLE 28 A continuously forming length of an unbonded fiber mat of 40% wool, 40% jute and sulfite cellulose, having a density in the range from 1 to 2 pounds per cu. it, was impregnated by confining it between carrying screen conveyers and running the assembly through an impregnating bath at about 100 to 120 F. The liquid of the bath contained by weight 8% of highly oxidized corn starch, 0.1% of nonionic wetting agent, 0.06% of CMC-HV, 1.6% of water-soluble urea-formaldehyde resin, and 0.07% aluminum sulfate. The ureaformaldehyde provides a final resin for imparting waterresistance to the finished fabric, and the alum is employed to adjust the pH for setting the resin. Excess liquid was expressed to leave 12 parts of the starch-plusresin to 88 parts of fiber. Then the mat was dried without restraint by running the wet mat over a bank of drying drums to expose each face alternately to the atmosphere.

EXAMPLE 29 To provide a control for Example 28, a similar operation was made with the same materials except that the CMC-HV was omitted. Table VH1 above gives some physical characteristics of the two bonded mats of Examples 28 and 29, showing that the migration of the binder is minimized by the very small amount of CMC-I-IV.

From the foregoing examples and explanation it is apparent that numerous changes and modifications may be made without departing from the spirit and scope of the invention as expressed in the appended claims.

I claim:

1. The method comprising continuously impregnating a moving web of loosely felted unbonded air-laid fiber mat with aqueous liquid from a supply thereof which has impregnating fiuidity at F.,' continuously expressing a portion of said liquid from the mat and returning the expressed portion to said supply, and drying the moving web by continuously exposing at least one face of the web to an evaporative atmosphere at a temperature in the range from 250 to 300 F., said aqueous liquid comprising at least 2.5% by weight of colloidal amylaceous binder which is substantially stable in viscosity on standing in aqueous colloid form and which is selected from the group consisting of oxidized starch, enzyme-hydrolysate of starch and dextrin, and said liquid containing up to 1.0% by weight of water-dispersible hydrophilic agent other than said binder which agent has greater viscosity characteristics in said liquid than said binder in said liquid under the conditions of drying the mat, the hydrophilic agent being selected from those which at a concentration in the range from 0.05% to 1.0% in the liquid and in combination with said binder in said liquid impart impregnating fluidity to the liquid at 80 F. and increase the viscosity of the liquid over that deriving from the binder under the conditions of drying such that by the presence of said hydrophilic agent the resulting viscosity minimizes the tendency of said binder to migrate during drying.

2. The method of claim 1 in which the hydrophilic agent has viscosity characteristics which increase with temperature.

3. The method of claim 2 in which the hydrophilic agent is methyl cellulose.

4. The method of claim 1 in which the web is dried by substantially identical exposures of both faces, whereby the resulting mat is substantially symmetrical with respect to its median layer.

References Cited in the file of this patent UNITED STATES PATENTS 1,043,988 Williams Nov. 12, 1912 1,574,896 Johnson Mar. 2, 1926 2,141,313 Osgood Dec. 27, 1938 2,250,681 Schwartz July 29, 1941 2,280,699 Grant Apr. 21, 1942 2,322,887 Schwartz Jan. 29, 1943 2,504,208 Locke Apr. 18, 1950 2,631,946 Schueler Mar. 17, 1953 2,653,140 Allenby Sept. 22, 1953 2,663,989 Schlatter Dec. 29, 1953 FOREIGN PATENTS 29,130 Great Britain Oct. 5, 1905 1904 468,031 Canada Sept. 12, 1950 

1. THE METHOD COMPRISING CONTINUOUSLY IMPREGNATING A MOVING WEB OF LOOSELY FELTED UNBOUNDED AIR-LAID FIBER MAT WITH AQUEOUS LIQUID FROM A SUPPLY THEREOF WHICH HAS IMPREGNATING FLUIDITY AT 80*F., CONTINUOUSLY EXPRESSING A PORTION OF SAID LIQUID FROM THE MAT AND RETURNING THE EXPRESSED PORTION TO SAID SUPPLY, AND DRYING THE MOVING WEB BY CONTINUOUSLY EXPOSING AT LEAST ONE FACE OF THE WEB TO AN EVAPORATIVE ATMOSPHERE AT A TEMPERATURE IN THE RANGE FROM 250* TO 300*F., SAID AQUEOUS LIQUID COMPRISING AT LEAST 2.5% BY WEIGHT OF COLLOIDAL AMYLACEOUS BINDER WHICH IS SUBSTANTIALLY STABLE IN VISCOSITY ON STANDING IN AQUEOUS COLLOID FORM AND WHICH IS SELECTED FROM THE GROUP CONSISTING OF OXIDIZED STARCH, ENZYME-HYDROLYSATE OF STARCH AND DEXTRIN, AND SAID LIQUID CONTAINING UP TO 1.0% BY WEIGHT OF WATER-DISPERSIBLE HYDROPHILIC AGENT OTHER THAN SAID BINDER WHICH AGENT HAS GREATER VISCOSITY CHARACTERISTICS IN SAID LIQUID THAN SAID BINDER IN SAID LIQUID UNDER THE CONDITIONS OF DRYING THE MAT, THE HYDROPHILIC AGENT BEING SELECTED FROM THOSE WHICH AT A CONCENTRATION IN THE RANGE FROM 0.05% TO 1.0% IN THE LIQUID AND IN COMBINATION WITH SAID BINDER IN SAID LIQUID IMPART IMPREGNATING FLUIDITY TO THE LIQUID AT 80*F. AND INCREASED THE VISCOSITY OF THE LIQUID OVER THAT DERIVING FROM THE BINDER UNDER THE CONDITIONS OF DRYING SUCH THAT BY THE PRESENCE OFF SAID HYDROPHILIC AGENT THE RESULTING VISCOSITY MINIMIZES THE TENDENCY OF SAID BINDER TO MIGRATE DURING DRYING. 